Various designs of mobile work platforms capable of vertically lifting personnel are known in the art. Telescoping mast personnel lift devices are commercially available from Genie Industries, Redmond, Wash., for example. In those devices, a base frame of fabricated aluminum supports an aluminum telescoping mast, including five or six stages interconnected by chains. When extended, the telescoping mast elevates an operator's cage that is designed for ground level entry. The operator's cage of this prior art lift design is shown in FIG. 1.
A cage assembly 10 is formed with a completely enclosed lower portion 12 and a completely enclosed upper bar 14 connected by a plurality of vertical connecting bars 16. Approximately at the lengthwise midpoint of vertical connecting bars 16, a plurality of horizontal safety bars 18a, 18b, 18c, and 18d are deployed. Horizontal safety bars 18a, 18b and 18c are permanently affixed to their respective next adjacent vertical connecting bars 16. In contrast, a horizontal access bar 20 is equipped with a set of two securing loops 22. Securing loops 22 enclose the vertical connecting bars 16 positioned adjacent to access bar 20, and securing loops 22 rest upon next adjacent horizontal connecting bars 18a and 18c. As a result, horizontal access bar 20 may be moved vertically in the direction indicated by arrow A to allow operator access to cage assembly 10.
This manner of operator access is awkward, requiring the operator to simultaneously lift access bar 20, pass under upper bar 14 and step over lower portion 12. The awkwardness of operator access to cage assembly 10 leads some operators to secure horizontal access bar 20 to upper bar 14. As a result, the operator will be able to have both hands free when gaining access to cage assembly 10. Such altered deployment of horizontal access bar 20, when continued during operation of the personnel lift, decreases the safety of the lift, however.
To lift cage assembly 10, an electric motor powers a hydraulic fluid pump to deliver working fluid pressure to a hydraulic cylinder. Since the hydraulic cylinder is attached to the base frame and the mast, extension of the cylinder results in elevation of the mast. A dual chain system operates to sequence the extension of the individual mast stages to achieve the desired height of cage assembly 10.
In conventional personnel lift devices, the operator may control (i.e., raise or lower cage assembly 10) with one hand. Specifically, the control box used with the personnel lift is designed such that actuation of a single control results in cage assembly 10 movement. As a result, the operator may raise or lower cage assembly 10 while leaning out over the edge thereof. This uneven distribution of the operator's mass during cage assembly 10 movement is a destabilizing factor that decreases the safety of the personnel lift.
Equipment production costs are affected by the amount of machining required. Operations, such as drilling holes in structures to permit bolt or screw access during assembly and the like, increase manufacturing complexity and therefore the time required for and the cost of such manufacturing. Masts of prior art devices, for example, have a plethora of holes machined therein to accommodate attachment of cable sheaves, studs of various types, brackets, braces, and the like as well as to permit mast assembly.
In addition, the mast of the prior art device features tracks, within which each mast stage travels to raise or lower the operator's cage. Each mast stage is equipped with a plurality of rollers to facilitate the movement of the mast stage within its tracks. This roller/track configuration requires a significant amount of machining. In addition, roller/track engagement may result in structural instability resulting from concentrated stress.
For maximum safety in operation, four removable outriggers, equipped with screw jacks on the outboard end thereof, should be deployed such that the base of the personnel lift device is level. This personnel lift device can, however, be operated without outriggers. Consequently, an operator faced with a single, discrete task may be tempted to forego outrigger use and attempt to complete the task using the lift device in an unsafe fashion.
A recognized problem with personnel lift devices is the difficulty in transferring them between essentially horizontal surfaces at varying heights (i.e., loading the lift from the ground onto the bed of a truck). A feature of the prior art apparatus previously under discussion addresses this concern. In that prior art design, a pivot point is adjustable to accommodate variations in vertical distance between the essentially horizontal surfaces. A stop with a pull pin is used to prevent a wheel associated with the pivot point from moving up the mast when the lift device is tilted. The wheel is permitted to move down the mast (through a roller/track system) as the lift device is being shifted horizontally at the new height. In this manner, the transfer operation may be carried out in reverse to lower the lift device to its original vertical level without any adjustment by the transferor. This feature is especially useful where the vertical transfer of the lift device constitutes temporary storage for transportation to another work site or until use thereof is again required. An analogous, dual pivot, slide block design is also commercially employed for this purpose. The battery compartment of each of these lift devices is disposed at a location that would interfere with this transfer process and must therefore be removed prior to transfer.
In a different design, a fixed set of wheels is located along the rear of the mast (i.e., the side of the mast opposite the side that is adjacent to the operator's cage). The set of wheels acts as the point about which the personnel lift device is pivoted when it is being transferred from one essentially horizontal surface to another. The position of the wheels along the mast is not adjustable and therefore the configuration represents the optimal design (i.e., requires the least force to effectuate the transfer) for transfers through one specific vertical distance only. Moreover, the battery pack compartment of this lift device is also disposed, such that it must be removed prior to transfer.
U.S. Pat. No. 4,709,784 describes an alternative loading facilitation mechanism, where a surface engaging pivot is deployed adjacent to a wheel on a single carriage. In this manner, the personnel lift device is pivoted about the surface engaging pivot until the lift is approximately horizontal. The surface engaging pivot is maintained in place on the higher horizontal surface throughout the pivoting operation by friction. At the end of the pivoting operation (i.e., when the lift is approximately horizontal), the wheel adjacent to the pivot is engaged and the lift can be rolled along the higher horizontal surface. The pivot/wheel assembly is mounted on a bracket and is height-adjustable through a mechanism including a plurality of adjustment holes located in a spaced-apart relationship along the side of the lift. The bracket has a key receiving hole therein capable of accepting a key, allowing the bracket to be affixed at a desired height when the key is placed through the bracket and one of the adjustment holes.